1. Field of the Invention
The invention relates to the manufacture of a high optical quality, transparent plastic sheet which can be used alone or in combination with other materials, particularly in laminated glazings where it is combined with a monolithic or laminated glass and/or plastic support, for example vehicle windshields, with the plastic sheet comprising at least one base layer of polyurethane having energy-absorbing properties.
2. Discussion of the Background
Plastic sheets capable of being used in laminated glazings have already been proposed. French Patent No. 2,398,606 describes a sheet having two layers, which are: a layer of a thermoplastic material which, in the laminated glazing application comprising a single sheet of glass, is an intermediate layer with energy-absorbing properties, and a layer of heat-hardenable material with anti-lacerating and auto-cicatrizing properties.
The intermediate layer with energy-absorbing properties is a thermoplastic polyurethane obtained from at least one aliphatic diisocyanate and at least one polyesterdiol or polyetherdiol, with the ratio of the equivalent NCO groups to the equivalent OH groups preferably being between 0.8 and 0.9. Glazing using such a two-layer sheet retains its good optical properties and the adhesion between the components under very variable conditions of temperature and humidity, but the bio-mechanical properties of the glazing and, in particular, the shock-resistance are not completely satisfactory. On the other hand, the two-layer plastic sheet can be rolled and easily handled without deterioration of its optical quality.
European Patent 0 133 090 further discloses a high optical quality, transparent sheet which can be used alone or in combination with other materials, particularly in the manufacture of the laminated glazings described above. This sheet comprises a layer formed in a continuous process by the reactive pouring onto a flat horizontal support, from which it is detachable, of a reaction mixture of an isocyanate component and a component with active hydrogens, in particular a polyol component. The isocyanate component comprises at least one aliphatic or cycloaliphatic diisocyanate or a diisocyanate prepolymer, with this component having a viscosity of less than approximately 5000 centipoises at +40.degree. C. The polyol component comprises at least one difunctional long polyol with a molecular weight of between 500 and 4000 and at least one short diol as a chain extending agent. Reactive pouring signifies pouring in the form of a layer or a film of a liquid mixture of the components in the monomeric or prepolymeric state, following by the polymerization of the mixture using heat.
Safety glazing using such a layer has good bio-mechanical properties under variable conditions of temperature and humidity.
The heat polymerization of the layer uses an industrial heat cycle, for example 20 minutes at a temperature of 120.degree. C. At the end of this industrial cycle, there is not total polymerization or aging of the layer. In fact, for the reaction to be complete, a much longer polymerization heat cycle must be used than the cycles used in industrial production, which results in a very long manufacturing line or otherwise much higher temperatures must be used with all the disadvantages which such can create: high energy consumption, yellowing of the layer, etc.
Increasing the amount of catalyst in the reaction mixture could also be considered, but this would produce a premature polymerization of the mixture in the device carrying the mixture to the support and, lastly, the layer obtained is not optically homogeneous.
One of the disadvantages of incomplete polymerization is the possibility of marking the layer. When at the end of the plastic sheet manufacturing line, for reasons of stocking and ease of handling, the sheet is rolled on reels or formed into stacks, using for this purpose an intermediate film which is generally polyethylene, when the sheet is unrolled or removed from the stack before use, marks or impressions left by the intermediate film are observed on the polyurethane layer with energy-absorbing properties (more simply designated as the "EA layer").
These marks cannot subsequently be totally removed, even when the plastic sheet undergoes a heat cycle for several minutes at a temperature above 100.degree. C., for example 10 minutes at 120.degree. C.
To overcome this disadvantage, European Patent Application 87 401 402.0 proposes subjecting the EA polyurethane layer obtained by reactive pouring onto a flat support as described above, or also by reactive spraying, with a state of polymerization advance corresponding to an NCO/CH ratio of below 0.7, a treatment using water in liquid or vapor form and preferably, when the water is in liquid form, using hot water at a temperature above 80.degree. C., to obtain a nil NCO/CH ratio (equal to zero). The NCO/CH ratio is the ratio between the height of the NCO peaks and the height of the CH peaks of the infrared spectrum of the EA layer upon exiting the polymerization area.
The water treatment enables the polymerization of the layer to be completed rapidly while retaining, and even improving, the mechanical properties of the layer, in particular its resistance to breakage and its stretch resistance.
When the layer is packed and/or stocked with the interposition of an intermediate layer in the form of stacked sheets, or in the form of a roll, after removal from the stack or unrolling of the layer, the layer is subjected to an adequate heat treatment, for example a cycle of at least one minute at a temperature of at least approximately 100.degree. C.
The hot water treatment just described does not entirely remove the marking of the layer due to the contact with the intermediate film, but it is no longer a persistent marking; it can disappear during a heat treatment of the layer, following the unrolling from the spool or the removal from the stack of sheets. The subsequent heat treatment can be carried out just before the assembly with the support to form the glazing.
The treatment with water in liquid or vapor form, however, requires an assembly line which is long to permit this treatment after the layer has reached a degree of polymerization corresponding to an NCO/CH ratio of below 0.7.